Hydrocarbons, including liquefied natural gas (LNG) and ethylene, may be used in a refinery, or other petrochemical setting, as an energy source or source material for various processes. Typically, one or more compressors may be used in the processing of such hydrocarbons. In particular, the propane and propylene compressors utilized for the processing of LNG and ethylene, respectively, are typically beam-style, multi-stage centrifugal compressors.
Generally, a beam-style, multi-stage centrifugal compressor includes a casing and a plurality of stages disposed therein, each stage including an inlet guide, an impeller, a diffuser, and a return channel that collectively raise the pressure of the gas or working fluid. A main inlet of the beam-style, multi-stage centrifugal compressor receives the gas flow from an inlet pipe coupled to the main inlet, distributes the flow around the circumference of the casing, and injects the flow into the first inlet guide disposed immediately upstream of the impeller of the first stage. The gas is drawn into the impeller from the first inlet guide and driven (or propelled) to a tip of the impeller, thereby increasing the velocity of the gas. The centrifugal compressor may also include a diaphragm assembly including all of the various components contained within the back half or downstream end of the compressor stage. The diaphragm assembly may form at least in part the gas flow path of the centrifugal compressor.
The diaphragm assembly may include a diffuser proximate the tip of the impeller and in fluid communication therewith. The diffuser is configured to convert the velocity of the gas received from the impeller to potential energy in the form of increased static pressure, thereby resulting in the compression of the gas. The diaphragm assembly further includes a return channel in fluid communication with the diffuser and configured to receive the compressed gas from the diffuser and inject the compressed gas into a succeeding compressor stage. Otherwise, the compressed gas is ejected from the gas flow path via a discharge volute or collector that gathers the flow from the final stage and sends it down the discharge pipe.
Applications, such as propane refrigeration or propylene units for LNG and ethylene, respectively, generally require one or more flow streams, generally referred to as sidestream flows, to be introduced into the centrifugal compressor at respective flow inlets other than the main inlet. These sidestream flows may be introduced through additional flanges added to or formed in the casing. The additional inlets required for the sidestream flow typically necessitate corresponding components including, for example, sidestream inlet plenums and sidestream scoop vanes, to mix the sidestream flow with the working fluid in the centrifugal compressor.
The mixing of the sidestream flow and the working fluid typically occurs in the inlet guide of the respective stage, immediately upstream of the impeller. Improper or insufficient mixing can lead to pressure and temperature stratification (i.e., non-uniform pressure and temperature fields). Such skewed pressure and temperature fields degrade the performance of the downstream stage, causing the operating pressures to fall short of the process requirements. Moreover, it is often desirable to have the ability to adjust the performance of the compressor to match the process requirements via movable geometry (such as movable inlet guide vanes or movable diffuser vanes). Generally, it is much more challenging to install movable geometry in a beam-style compressor because of the limited space in which to install the drive mechanisms and linkages.
What is needed, then, is an efficient system including a compressor configured to provide for a working fluid and sidestream flow mix having a substantially uniform temperature and pressure field, and further configured to allow for the facile installation of movable geometry to provide for the tuning of the compressor for varying process requirements.